Does anyone know how to remove this crank arm? Or if it can be removed? The bike was ~$250 and seems to have a lot of 'don't take this apart' engineering, I'd like to soak the chain rings in rust remover without the arm interfering. The interface is also just a bit loose. It's not clear if there's any way to tighten that gear-looking around the square.
Sorry, those chainrings are riveted in place. You just have to replace the whole crank arm.
You could drill out the rivets (and later replace them with button-head hex screws) so that you could take off the chainrings. But this would be more effort than its worth. And it's not like you could easily find replacement chainrings anyway.
As @criggie notes, there's not nearly enough rust to make this worth it. Scrubbing the chainrings with chain cleaner and a brush should take off most of it. Those 'rust' marks are places where the chain abrades when the gears shift, any paint you put on them will be scraped off the next time you shift.